Flexographic and rotogravure printing processes have been widely practiced in the past using solvent-based inks. More recently, environmental, fire hazard, toxicological and petrochemical supply concerns have provided a growing impetus for the development of water-based inks to be used in these printing processes. Although these efforts have resulted in many commercially successful inks for corrugated and paper applications, development of suitable water-based inks for printing on plastic substrates has met with somewhat less progress. This is understandable when one considers that the surface energy of typical plastic substrates such as polyethylene and polypropylene are much lower than water, resulting in poor wetting of these materials by water-based inks. This problem has been partly overcome in the art by inclusion in the ink composition of at least one organic co-solvent which reduces the ink's surface tension. Thus, for example, low slip polyethylene for sanitary overwrap applications has been successfully printed upon using commercial water-based inks.
However, there still remains room for improvement in coating other low surface energy plastics such as high slip polyethylene, which contains fatty amide slip additives. This latter substrate is quite difficult to print upon using conventional water-based inks, the major problems being pinhole formation and lack of adhesion of the ink to the substrate. Furthermore, unlimited addition of co-solvents can not be considered a panacea in this case, or, for that matter, in general, since there are at present a number of Environmental Protection Agency (EPA) regulations and guidelines stemming from the Clean Air Act which limit the volatile organic compound (VOC) content of the ink compositions. For example, a so-called "compliance water-based ink" contains a volatile portion which consists of at most 25 volume percent of VOC. Therefore, there is a need in the art for flexographic and rotogravure water-based ink compositions which utilize minimum amounts of organic co-solvents while still providing good wetting and printing characteristics on low surface energy plastic substrates.